Without further explanation, "You won't be a good fit" is as generic as it could be, and it is a kiss of death, unless you are assertive enough to ask questions (and persevere in doing so until you get answers).
As you have not much to lose anymore, you can take the risk to ask anything, like
- What would it take to fit in better?
- Do you see a mismatch with the function, the team or the company culture?
- Is there a different team or function in your company in which I would fit better?
- Do you know a company in your industry whose company culture would fit me better?
Beware you are in an exceptional position now: you do have access to a person who knows the industry. That person sees him/herself as the one who turned you down and would be much happier to see him/herself as the one who gave you advice.
Do both of you a favor and act now.
My optimism gets a lot of negative feed back, mainly from people that are afraid the hiring manager or recruiter you contact might not appreciate being bothered or even questioned by you. I aggree this is a risk, but it is one worth taking.
It is a risk because they might not want to discuss the reason why you are not hired.
(It could be embarrassing or even illegal, e.g. discrimination) Therefore ask what you could do better, never ask what is wrong. The first question I suggested is the most dangerous.
It is also a risk
because some people only see you as a resource, which accidentaly is human.
These recruiters and hiring managers don't care about you, not when you get the job and even less when you don't.
If they remember you needlesly bothering them in the past, they will reject you in the future.
But the riks is worth taking
because others see you as a human, which might become a resource.
They not only like to help humans, they also know humans are capable of improving themselves using feedback.
If they remember you valuing their feedback in the past, they will valuate your effort to fit in in the future.
I don't mind burning bridges to the first type of recruiter while building bridges to the second type. They will propose me jobs in which I will be productive and happy.